logger: remove unnecessary lock for log rotation

The logger process runs with one thread, so no need for
pthread_mutex_lock.

Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
Signed-off-by: Liu Yuan <namei.unix@gmail.com>
md: remove unnecessary sd_err

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>

Conflicts:
	sheep/md.c

Conflict was resolved by Hitoshi Mitake.
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
sheepfs: add -lpthread to the linker option

Because libsheepdog wraps pthread reader writer lock, the flag is
required for building sheepfs too.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>